This invention relates to a sintered silicon carbide ceramic material having very high electrical resistivity, and a process for making such a material. Such material is suited for use in manufacture of a body suitable for use as a heat conducting electrical insulator for electrical and electronic devices. Such a heat conducting electrical insulator may be referred to as a substrate. This invention particularly relates to sintered, substantially homogeneous, silicon carbide ceramic bodies which have an electrical resistivity of at least about 10.sup.8 Ohm cm, and to a process for sintering of a ceramic body having such electrical resistivity.
For many devices in the electrical and electronics industry, there is a present and growing need for material of high thermal conductivity and high electrical resistivity. Such materials can then serve to conduct electrically generated heat to the ambience, while acting as a barrier for electrical currents to that same ambience. Additionally, it is often very desirable that the materials be readily formed into complex shapes at low cost; for ceramic materials, this often requires pressureless sintering.
The semiconductor industry has progressed rapidly in recent years. The number of circuit constituents such as semiconductor chips are being formed in increasingly higher density on an electrically insulating substrate. To meet the demand for devices of greater capacity and smaller size, there is continued development of large scale integrated circuits and very high speed integrated circuits of increasing junction density. These trends result in increased demand for electrically insulating substrates of higher thermal conductivity and/or higher thermal diffusivity to remove the heat generated during operation of these circuits.
Conventionally there is employed for the substrate an alumina (Al.sub.2 O.sub.3) sintered body. A sintered alumina body typically has an electrical resistivity of about 10.sup.14 Ohm cm at 25.degree. C. and a thermal conductivity of about 20-25 W/mK (watts per meter kelvin). The density and speed of operation of new circuit design is limited by the thermal conductivity of these alumina substrates.
Characteristics of an ideal substrate material include the following:
(1) high electrical resistivity; PA0 (2) high thermal conductivity; PA0 (3) coefficient of expansion close to that of silicon; PA0 (4) high mechanical strength; PA0 (5) metallizable, and PA0 (6) low dielectric constant at frequencies of 10 MHz (megahertz) or more. PA0 (a) up to 1.0 percent uncombined carbon; PA0 (b) 0.3 to 1.0 percent boron; PA0 (c) 0.03 to 0.8 percent nitrogen; PA0 (d) and a balance of silicon carbide; PA0 (a) forming a shaped body having a density, before sintering of at least about 1.45 g/cm3, the shaped body composed essentially of: PA0 (b) sintering said shaped body under substantially pressureless conditions in a nitrogenous atmosphere at a temperature of about 2250.degree. C. or greater, for a time sufficient to produce a sintered body having a density of at least 2.95 g/cm.sup.3 and an electrical resistivity of at least 10.sup.8 Ohm cm at 2500.degree. C.
A sintered body of silicon carbide has a coefficicent of linear thermal expansion of about 3.7.times.10.sup.-6 /.degree.C. which is near to that of silicon which is about 3.3.times.10.sup.-6 /.degree.C. Additionally, the variation with temperature of the coefficient of thermal expansion of silicon carbide is very close to that of silicon throughout the range of 25.degree.-1000.degree. C. The chemical and physical properties of silicon carbide make it an excellent material for high temperature structural applications. These desirable properties include good oxidation resistance and corrosion resistance, high thermal conductivity compared to alumina, low expansion coefficient compared to alumina, high resistance to thermal shock, and high strength even at an elevated temperature. These properties are achieved by known bodies of sintered silicon carbide, including those produced by pressureless sintering. However, these known silicon carbide bodies exhibit electrical resistivities of typically 10.sup.2 and at most up to about 10.sup.6 Ohm cm which are well below those required for use as substrates in integrated circuits.
No one has been able to make an electrically insulating silicon carbide grain. The single crystal resistivity of various types of silicon carbide crystals is given in "Silicon Carbide-1973, Proceedings of the Third International Conference," Miami, Fla., September 1973, edited by R. C. Marshall et al, University of South Carolina Press, Columbia, SC. None of the 6H alpha-phase, single crystal poly types, characterized at Appendix II at page 673, exhibited an electrical resistivity of greater than 95 Ohm cm at 25.degree. C.
From U.S. Pat. No. 4,370,421, there is known an electrically insulating, high thermal conductivity substrate consisting of silicon carbide as its principal component. This material is formed by adding 0.1 to 3.5 weight percent of beryllium oxide powder, calculated as beryllium, to silicon carbide powder which additionally contains aluminum, boron and free carbon components in amounts restricted to, at most, 0.1 weight percent, at most, 0.1 weight percent and, at most, 0.4 weight percent, respectively. Given the toxicity of beryllium oxide, it is desired to provide a sintered silicon carbide body suitable for use as a substrate which does not contain beryllium compounds or employ beryllium oxide in its manufacture.
Bodies of silicon carbide have heretofore been produced by reaction bonding (also known as reaction sintering) and hot pressing. Reaction sintering involves use of silicon impregnants to upgrade the density of silicon carbide through reaction with excess carbon in the substrate. Reaction sintering is useful for many applications but is undesirable where excess silicon exuding from the silicon carbide body would be detrimental (e.g. high temperatures in excess of 1400.degree. C.). Reaction sintered silicon carbide bodies typically exhibit low electrical resistivity; e.g., 0.2 Ohm cm. Hot pressing (the production of high density silicon carbide ceramic bodies by simultaneous application of heat and pressure) is impractical for complex shapes because the pressure required (typically of the order of greater than 1000 psig) cannot be uniformly transmitted to all parts of the required mold, which results in a deformed body. Also, difficulty may be encountered in removing a hot pressed part from a complex mold.
U.S. Pat. No. 3,960,577 describes a hot pressed silicon carbide body having a maximum room temperature electrical resistivity of 50 Ohm cm. This hot pressed body has a density of at least 98 percent of theoretical density of silicon carbide and is substantially non-porous. This material is hot pressed at 5,000 to 10,000 psi at 1950.degree. C. to 2050.degree. C. from a mixture of submicron beta phase silicon carbide, sufficient boron-containing additive to provide 0.3 to 3.0 percent boron and 3.5 to 10 percent of Si.sub.3 N.sub.4. The sintered product is described as having sufficient nitrogen atoms accommodated in the lattice of silicon carbide to make it conductive with the boron additive in solid solution in the silicon carbide.
U.S. Pat. Nos. 4,312,954; 4,124,667; 4,346,049; 4,179,299; 4,135,938; 4,172,109; 4,123,286; 4,135,937; 4,144,207; 4,207,226, and 4,237,085 disclose pressureless sinterable silicon carbide compositions that may contain, in some instances, up to 5 percent uncombined carbon in the final sintered silicon carbide product and, in other instances, up to 6 percent uncombined carbon in the final sintered product and which are prepared from silicon carbide, boron carbide and a free carbon source. U.S. Pat. No. 4,525,461 describes a pressureless sintered silicon carbide/graphite/carbon composite ceramic body, of which certain embodiments have very low electrical resistivity. A hot pressed body formed according to U.S. Pat. Nos. 4 135 937 and 4,135,938 may contain up to 15 percent additional carbon (beyond that in the original particulate silicon carbide) derived from graphite or carbonized organic composition. In U.S. Pat. No. 4,135,938 the belief is stated that most of the additional carbon is chemically combined with the silicon carbide and additive compound (for example, BP, BN or AlB.sub.2). None of these Patents, excepting U.S. Pat. No. 4,370,321, disclose a sintered predominantly silicon carbide ceramic body having sufficient specific electrical resistivity for use as an electrically insulating substrate as is required for integrated circuits.
Thus, there remains a need for a silicon carbide material which is prepared from non-toxic ingredients and which can be cost effectively manufactured into complex shapes desired for electrically insulating devices.
The terms "free" and "uncombined" as used herein are synonymous and mean "not chemically combined." For example, uncombined carbon in a sintered body according to the present invention is not chemically combined with, for example, silicon to form silicon carbide.
In this abstract, specification and claims, unless otherwise indicated, all quantities, proportions and ratios are stated on a weight basis.